The invention relates to a diagnostic radiology apparatus for producing layer images of a radiography subject with a patient support, having a measuring arrangement for irradiating the radiography subject from different directions, comprising a radiation source which emits a radiation beam which penetrates the layer to be examined, and a radiation receiver which is located behind the radiography subject relative to the source and which supplies electrical output signals corresponding to the radiation intensity measured, and having a computer connected thereto for calculating from the output signals of the radiation receiver the attenuation values of image points of the body layer irradiated. An apparatus of this type is described in U.S. Pat. No. 4,065,397.
Known diagnostic radiology apparatus of this type, so-called computer tomographs, have x-rays tubes as radiation sources. They emit a Bremsstrahlen spectrum with wide spectral distribution. Even after normal preliminary filtration, this is still so wide that when the patient is irradiated there is a hardening of the rays which is dependent upon the thickness and the composition of the object irradiated. For producing an image from the output signals of the radiation receiver, as a result of this variable hardening of the rays which occurs in the body of the radiography subject,
(1) in the interest of uniformity of the image and above all for a quantitative image evaluation, correction must take place in accordance with this hardening,
(2) variable energy dependency of the radiation detectors must either be avoided or likewise be corrected, and
(3) fluctuations in the average energy of the spectrum emitted from the radiation source (inclusive of preliminary filtration) must either be kept sufficiently small or likewise be corrected.
According to a prior patent application (corresponding to German Pat. No. 28 31 038.5) a rule is specified according to which a diagnostic radiology apparatus of the above-mentioned type is constructed such that the radiation quality within the radiation receiver or in its immediate vicinity can be measured, with the result that the corrections described can be made accurately.
In a development of the invention according to prior patent application and Pat. No. 28 31 038.5, the invention is based on the fact that when a single absorption body is used, the measurement results are still always dependent upon the radiation hardness derived from the correction detectors which output is also dependent on said radiation hardness.
This is obtained according to the prior patent application (and Pat. No. 28 31 038.5) in that there are provided, in the correction detectors, absorbers for the radiation, connected in series therewith, with the result that the average radiation energy of the radiation received can be determined from the output signals of these detectors. The output signals of these detectors depend on the quality of the radiation received in each case with the result that this quality can be determined from them. Therefore, they can be used to correct the measured signals.
It is true that U.S. Pat. No. 4,065,397 already describes a diagnostic radiology apparatus of the above-mentioned type where there is correction of the type described above; however, with this known diagnostic radiology apparatus this correction is made by a function stage wherein the correction function is stored. The respective radiation quality is not measured.